1. Field of the Invention
The present invention relates to an image heating device for heating an image on a recording material, and more particularly to an image heating device adapted for use as a fixing device in an image forming apparatus.
2. Related Background Art
For heating an image on a recording material there is widely employed a system of contacting a recording material with a heating member maintained at a predetermined temperature, as exemplified by the heated roller system.
Such heating system requires a certain warm-up time until the heating member reaches the predetermined temperature after the start of power supply thereto.
For reducing the warm-up time, there are generally two methods, a first method being to increase the power of the heater constituting the heat-generating member, while a second method being to reduce the thickness of the heating roller in order to reduce the heat capacity thereof.
However, the first method is associated with a drawback of lack in electric power in the entire image forming apparatus, since, even though an enough electric power can be supplied to the heater at the warm-up stage, the electric power has to be supplied to other units of the apparatus in addition to the fixing unit upon operation of the ordinary image forming. The second method shortens the warm-up time, but is associated with the following drawback. In an image forming apparatus available for recording sheets (as recording material) of various widths, the heat amount distribution of the heater is designed for the recording sheet of the largest width. Consequently, if the recording sheets of narrower width are made to pass in succession, the end portions of the heating roller becomes very hot, due to the lack of heat dissipation to the recording sheet, in comparison with the central portion of the roller where the recording sheet passes, thus eventually giving rise a failure of the device in some cases. This is the reason that it is difficult to dissipate heat in the longitudinal direction of the roller due to the reduced thickness of the roller.
There is also another method of adopting two heaters, and energizing both in the warm-up stage while energizing only one in the ordinary image forming operation, but such configuration is still associated with the above-mentioned drawback of temperature rise in the end portions of the roller when the narrow recording sheets are used in succession.